Polyethylene terephthalate (PET), an aliphatic/aromatic polymer, is a well known thermoplastic material commonly used in the production of filaments for a variety of applications. Because of its excellent mechanical and fiber-forming properties such as tensile strength and loop strength, PET is especially useful in industrial applications such as for the production of fabrics for filtration and the reinforcement of various structures. Moreover, the monofilaments produced from PET are frequently woven into support belts or fabrics for transporting and dewatering paper sheets on paper machines.
However, this application, as many other applications for which PET monofilaments are suited, provides a rather harsh environment which often subjects the PET to high temperatures and harsh chemicals including hot water, chemicals having a high pH and so forth. When exposed to wet or dry high temperature conditions and/or to harsh chemical environments for significant periods of time, the PET filaments are known to degrade. Unfortunately, while stabilizing additives and stabilization techniques can be employed to extend the useful life of the PET, such additives and methods prolong the life of the PET filaments for only a relatively short period of time, and the PET filaments are eventually degraded under the conditions described hereinabove. Thus, because of its susceptibility to thermal, hydrolytic and other chemical degradation, the usefulness of PET filaments is particularly limited.
Similarly, polyphenylene sulfide (PPS) is a thermoplastic polymer which is also known to have use in the production of filaments and monofilaments for a variety of applications. In contrast to PET filaments however, PPS filaments have excellent thermal, hydrolytic and chemical resistance properties and therefore, may be particularly useful for the production of monofilaments for woven fabrics which are exposed to high temperatures and harsh chemical environments. Unfortunately, the useful applications for these PPS filaments are somewhat limited due to the relatively high cost of the material and its relatively poor mechanical properties. More particularly, PPS tends to be brittle. Notably, the tensile strength of PPS is generally about one-half that of PET, while the loop strength of PPS is about 50 percent lower than that of PET. This lower tensile strength and loop strength has been known to create processing problems when PPS filaments are woven into fabrics. Accordingly, a need exists for a monofilament which provides at least some of the excellent physical properties of conventional PET monofilaments, but which will have longer life in wet or dry environments at high temperatures or in environments with harsh chemicals than will filaments of conventional PET.
PPS has been blended with PET and with various other polymers as described in the following patents although monofilaments of PPS/PET blends have not been known heretofore. For example, in Salee U.S. Pat. Nos. 4,251,429 and 4,284,549, PPS was blended with a polymer prepared from an aromatic dicarboxylic acid and a bisphenol in order to enhance the hydrolytic stability of the blend. More particularly, the polymer was prepared by reacting bisphenol A with isophthaloyl chloride and terephthaloyl chloride to obtain a linear fully aromatic polymer. The resultant blend is unsuitable for filament extrusion however, in view of the blending process employed and the tensile strength of the molded product. Specifically, the blend was prepared by tumble blending the linear aromatic polymer with the PPS for up to one hour and subsequently milling at about 249.degree. C. (480.degree. F.). The step of blending was carried out before the step of molding. In addition, the tensile strength of the materials molded from the polymer blend is about 10,000 psi, approximately a factor of 10 less than the desired tensile strength of monofilaments suitable for use in most applications.
Similarly, in Salee U.S. Pat. No. 4,305,862, PPS was blended with fully aromatic polymers prepared from a aromatic dicarboxylic acid and a bisphenol. This blend was used to produce molded products and to prepare materials with improved dielectric strength. The resin blend and the blending process are unsuitable for extrusion of monofilaments, however.
In addition, Froix U.S. Pat. No. 4,276,397 relates to blends of PPS and fully aromatic polymers which blends were again used for molding products. Notably, the tensile strength of the molded products ranges from about 8400 psi to about 16,000 psi, again far less than the desired tensile strength of monofilaments. Additionally, the elongation of the products was less than 3 percent. Accordingly, these blends would be unsuitable for extruding filaments.
Still further, Cohen U.S. Pat. No. 4,140,671 discloses blends of PPS and polybutylene terephthalate (PBT) with glass fibers. This blend also contains flame retardants and talc. While the blends in Cohen are noted as being produced by extrusion, they are used for molding products, not monofilaments. In fact, the tensile strength of the molded products in Cohen is about 8,000 psi to 14,000 psi, again well below to desired tensile strength for monofilaments. Moreover, because of the glass fibers and talc in the blend, it could not be used for fibers or filaments.
Some blends of PPS with other polymers have been used to produce filaments; however, the mechanical properties of the filaments formed were significantly improved. For example, Baker et al. U.S. Pat. No. 4,755,420 discloses filaments made from blends of PPS and nylon 66. It was found that the brittleness of the PPS was reduced by the addition of 6 percent nylon 66. However, as the level of nylon 66 increased, abrasion resistance of the filament was decreased even though toughness increased.
In Ballard U.S. Pat. No. 4,610,916, filaments from blends of PPS and ethylenetetrafluoroethylene (ETFE) were disclosed. Again, the brittleness of the PPS was significantly reduced, as shown by the increase in the loop strength of the material; however, tensile and knot strength of the PPS were not significantly improved, even with increased levels of ETFE.
Still further, Skinner et al. U.S. Pat. No. 4,748,077 discloses filaments made of blends of PPS with polyethylene, polypropylene, polyethylene/propylene copolymers, polymetaxylylene adipamide and polyvinylidene fluoride. However, it is noted that increased levels of these materials in PPS did not improve the physical properties of the filaments.
In Smith U.S. Pat. No. 5,162,151, filaments made from blends of PPS with vinylidene fluoride/hexafluoropropylene copolymer were disclosed. This copolymer also did not improve the mechanical properties of the filaments.
Notwithstanding the above patents, attempts have been made to blend PPS and PET in order to improve the mechanical properties of PPS or the thermal and chemical resistance properties of PET in molding applications. However, because PPS and PET are not fully compatible, all of these attempts have met with unsatisfactory results unless a compatibilizing polymer or reinforcing filler was added to the blend. In fact, at least two patents expressly indicate that blends of PPS and PET must be formed using various compatibilizing agents, and even when such compatibilizing agents are employed, the formation of useful blends of PPS and PET requires extraordinary effort. Notably, the blends were prepared by kneading or by the use of a twin screw extruder.
In Nakata U.S. Pat. No. 4,997,866, PPS was blended with PET, PBT and polycyclohexanedimethyl terephthalate (PCT) to again produce molded products, not monofilaments. However, as noted therein, physical blends of these polyester resins with PPS were found to be unsatisfactory because of problems with compatibility. In order to prepare satisfactory blends for use in the molded products being produced, it was necessary to use compatibilizing agents such as unsaturated polymers. As shown in Examples 4, 5 and 6 (Table 2) of the Nakata patent, a blend of PPS and PET devoid of compatibilizers was determined to be unsatisfactory.
In Kubota U.S. Pat. No. 5,218,043, blends of PPS with PBT and PET were disclosed. Again, as noted therein, blends of PPS with these polymers were found to be unsatisfactory for molding applications because of poor resin compatibility. In order to prepare satisfactory blends for molding, it was necessary again to use compatibilizers such as vinyl or allyl compounds containing an epoxy group. Moreover, blends without these compatibilizers were found to have unsatisfactory mechanical and physical properties.
Finally, when compatibilizers are not utilized, fillers or reinforcing fibers are necessary to provide sufficient strength to the products being molded. For instance, Chacko U.S. Pat. No. 4,689,365 discloses blends prepared from PPS and polybutylene terephthalate, nucleated PET or mixtures thereof as well as reinforcing fibers. Notably, the blends were prepared by first extruding the components and then molding the blend into a product. Unfortunately, this blend cannot be used for the extrusion of monofilaments because of the addition of glass fiber and other types of reinforcing fillers. The two-step blending process (blending and then molding) is also unsuitable for extrusion of monofilaments.
Thus, it will be appreciated that no attempts have been made to blend PPS with PET, or high temperature polyesters, with or without compatibilizers, in order to produce extruded filaments having at the same time thermal, hydrolytic and chemical resistance superior to filaments of PET, or high temperature polyesters, and physical properties superior to filaments of PPS. That is, heretofore, monofilaments have been produced which comprise only PPS or only polyester, not a blend of the polymers. Of course, some of these monofilaments included additives to stabilize and otherwise strengthen the monofilaments, but none used a blend of PPS and PET and/or high temperature polyesters. Because of the brittleness and relatively poor fiber-forming properties of PPS and the lower thermal, hydrolytic and chemical resistance properties of PET or high temperature polyesters, these "pure" PPS or "pure" PET or high temperature polyester monofilaments are limited in their applications and may not be suitable for demanding fabric designs and/or environments where high temperature and harsh chemicals may be present.
Accordingly, the production of an extruded monofilament comprising a blend of PPS and PET and/or high temperature polyester which has improved mechanical properties over conventional PPS monofilaments and improved thermal, hydrolytic, and chemical resistance properties over conventional PET or high temperature polyester monofilaments is highly desirable, and such a monofilament with these useful and superior properties would be highly unexpected, especially in view of the teachings of Nakata U.S. Pat. No. 4,997,866 and Kubota U.S. Pat. No. 5,218,043 which indicated that such useful properties could not be obtained from PPS/PET blends.
It should also be noted that high temperature polyester resins, having a melting point above about 260.degree. C., have been found to be suitable for use in monofilaments for paper machine fabrics. Such high temperature polyester resins include, but are not limited to poly-(cyclohexanedimethylene terephthalate) and poly(cyclohexanedimethylene terephthalate/isophthalate). The use of poly(1,4-cyclohexanedimethylene terephthalate/isophthalate) and its suitability for use in paper machine fabrics is essentially described in Eagles U.S. Pat. No. 5,169,499. It is referred to as poly(1,4-cyclohexanecarbinyl terephthalate) in this patent. Specifically, the Eagles patent discloses the use of a resin comprising carboxyl groups which are hindered by a moiety selected from cyclic aliphatic and branched aliphatic glycols and is noted as having excellent hydrolytic stability. However, this high temperature polyester resin has lower dry heat (thermal) stability as compared to conventionally stabilized PET.
Notwithstanding these improvements, a need still exists, as a result of the deleterious conditions which paper machine fabrics are subject to during the paper making process, for resin blends which may produce suitable monofilaments having improved resistance to hydrolytic, thermal and chemical degradation.